(1) Field of Invention
The present invention relates to the field of photonic quantum information systems specifically those using unpolarised photon sources. The present invention is particularly intended for use in quantum communication. The present invention also extends to photon emitters and methods for outputting photons.
(2) Description of Related Art
In quantum communication systems, information is transmitted between a sender and a receiver by encoded single quanta, such as single photons. Each photon carries one bit of information encoded upon a property of the photon, such as its polarisation, phase or energy/time. The photon may even carry more than one bit of information, for example, by using properties such as angular momentum.
Quantum key distribution which is a technique for forming a shared cryptographic key between two parties; a sender, often referred to as “Alice”, and a receiver often referred to as “Bob”. The attraction of this technique is that it provides a test of whether any part of the key can be known to an unauthorised eavesdropper (Eve). In many forms of quantum key distribution, Alice and Bob use two or more non-orthogonal bases in which to encode the bit values. The laws of quantum mechanics dictate that measurement of the photons by Eve without prior knowledge of the encoding basis of each causes an unavoidable change to the state of some of the photons. These changes to the states of the photons will cause errors in the bit values sent between Alice and Bob. By comparing a part of their common bit string, Alice and Bob can thus determine if Eve has gained information.
A photonic quantum computer or logic device is a device where gate operations are performed upon the encoded states of a number of single photon pulses in order to carry out some computation task. The single photon pulses will be provided either by an array of single photon sources or, alternatively using repeated emission from a single photon source with appropriate delays. In order for the logic gates to function properly it is often necessary for the photons to have the same polarisation.
Photonic quantum information systems are typically sensitive to the polarisation of the light used. For example, many photonic quantum communication systems encode information on the polarisation of the photons by applying a rotation to an initial polarisation state. However, this can only be done if the initial polarisation state is known. In other quantum communication systems, the information is encoded upon the phase of the photons in an interferometer. The components in the interferometer will often be sensitive to the polarisation of the photons. For example, the phase shift introduced by the phase modulator depends upon the polarisation of the photons. Thus photons in different polarisations will experience different phase shifts when passing the phase modulator.
To overcome the polarisation dependence of such systems, it is common to linearly polarise the light using a polarising filter. However, this has the disadvantage of reducing the efficiency of an unpolarised source. If for example the source is randomly polarised, this will reduce the bit rate by 50%. This problem is of particular concern for single photon sources where this loss cannot be compensated by increasing the intensity.